1. Field of the Invention
This invention pertains to acoustical well logging and more specifically to the development of an acoustical well log where the detected reflections are displayed in accordance with frequency response.
2. Description of the Prior Art
Acoustic well-logging employs in an electronic package or sonde lowered and raised using a cable operating in a borehole, an electro-mechanical transducer transmitter and a similar receiver or receivers vertically spaced apart therefrom. A receiver detects the returning acoustical impulses resulting from the operation of the transmitter and provides a signal suitable for revealing information about the adjacent casing, borehole and formation therebeyond.
Typically, the transducer transmitter is cycled or pulsed at a rate of many times per second, such as at 20 pps. Although recording is sometimes done in the sonde, it is usual for the detected or received impulses to be transmitted up a suitable communication-line part of the cable supporting the sonde for processing and displaying or recording at the surface contemporaneously with the development of the detected signal, such recording producing a permanent record for later evaluation.
Log information can be of tremendous utility; however, there is a constant effort in the industry to develop different types of logs, to refine the development of logs, and to improve the display and interpretation of logs. The main purpose of this on-going effort is a result of the inability to optimize the potentially tremendously valuable information that logs contain. This inability is a result of the errors that occur in log development, errors in their recording and errors in their interpretation, thereby distorting or obscuring the useful information. These errors are introduced by numerous conditions including, but not limited to, the conditions of the transmission path for the developed information, interference of transmitted signals with each other and conditions in the borehole and its environment.
Although with laborious analysis, many acoustical logs including various distortions and apparently obscured data can be processed to yield most of their valuable information, it has been difficult and therefore quite expensive to achieve such processing. Moreover, such processing is usually done after the recording act. If it were available immediately with the development of the log, this time-saving effect would be of itself a tremendously valuable result.
The normal acoustic log is developed from the production of an electro-mechanical signal which produces a wave front into the adjacent borehole and the formation beyond, normally using a transducer excited by a sine wave or pulses. A receiver operating in conjunction therewith detects and produces a mechanical-electrical signal which is proportional to the transmitted wave as changed in character by the environment intervening between the transmitter and the receiver. This resulting waveform is then displayed and/or recorded. The primary information has traditionally been in the amplitude of the recorded waveforms.
An advancement in the art of interpreting acoustic logging information known as microseismographics introduced the concept of eliminating some of the detected frequency spectrum where distortions were great, both at low and high frequencies, and integrating the resulting waveforms. Hence, the higher amplitude portions of the waveforms having the quickest rise and fall rates produced certain information when displayed in this integrated fashion that were not evident in the original form. This technique is described in U.S. Pat. No. 3,314,498, Anderson, et al. Related logging systems are also described in U.S. Pat. Nos. 3,302,165 and 3,393,404, also Anderson, et al., all of which are incorporated herein by reference for all purposes.
Another improvement of acoustic well-logging in the prior art was the use of complex pulses for exciting the transmitter transducer, such pulses comprising a plurality of different signal frequencies. The use of complex pulsing is described in an article appearing in the April, 1966 Journal of Petroleum Technology entitled "Some Effects of Frequency Upon the Character of Acoustic Logs", pages 407-411, by P. E. Chaney, Jr., C. W. Zimmerman and W. L. Anderson, which is incorporated herein by reference for all purposes. By experience it has been found that some frequencies transmitted and produced certain revealing detection results better than other frequencies. Hence, the character of the acoustic log could be enhanced by using those frequencies that produced the most revealing results.
However, heretofore it has not been recognized that there is useful logging information that is contained in the frequency content of the received signals that is not contained in the amplitude content of the same received signals. As previously mentioned, it is desirable to transmit signals that result in the most revealing received signals and to process such received signals to eliminate extraneous and interfering elements from the log. To do this, utilizing the useful information included in the frequency content of the logging signal has resulted in a new type of well log. In a preferred display format, data is displayed for convenient analysis by its content at various frequency "windows".
Therefore, it is a feature of the present invention to provide an improved acoustical logging of a borehole by displaying the received results in accordance with its frequency content.
It is another feature of the present invention to provide an improved acoustical logging of a borehole by performing spectrum analysis on detected acoustical signals as they are developed and displaying the frequency information in association with depth information.
It is still another feature of the present invention to provide an improved acoustical logging of a borehole wherein the acoustical information is transmitted to the surface for displaying and/or recording by correcting the received signals for attentuation differences at the various frequencies and then analyzing the signal by frequency content.
It is yet another feature of the present invention to provide an improved acoustical logging of a borehole by developing, by frequency content, two signals from spaced apart receivers and displaying the amplitude difference at a plurality of frequencies in association with depth information.